In this paper, we report the results on the influence of a top oxide layer on the He-cavity formation in silicon samples. Si samples with top oxide layers of different thickness together with the pure silicon (used as reference) were implanted at room temperature with 1 MeV 3He at a dose of 5 × 1016 He/cm2. After implantation, the oxide layer was removed by using HF solution. Cross-sectional transmission electron microscopy (XTEM) was used to study the induced cavities followed by annealing. The results show that He implantation induces a well-defined-damaged layer with a thickness of about 180 nm in the reference Si. The defects are mainly made up of big cavities in the middle of the projected range and a large population of smaller ones both towards the surface and into the bulk. Very few stacking faults and dislocations loops can be seen around this band. The creation of cavities in Si with a 2.3 μm oxide layer, however, is a little different. Regions containing a chain of the biggest cavities surrounded by few smaller ones have also been found. Unexpected results are obtained in Si with a 1.2 μm oxide layer. In this case, only a monolayer of big cavities is observed at the depth corresponding to the projected range. The results are discussed in combination with SIMS measurements and SRIM simulations.